Should be a tag somewhere on the rear that contains a bunch of numbers. On mine it was on the front section close by the pinion. If you can find it, post the info and someone here should be able to decode it.
It should be a 9" as stated above. One way to tell a 9" from an 8" is by looking at the two lower diff case nuts/studs. If you can get a socket on them, the rear is an 8". the 9" has a hump in the case, protruding downward; you won't be able to get a socket on the 9".